Using first-principle calculations, we compare the quantum efficiency and stability of Cs-GaN planar model and Cs-GaN nanowire model. The results show that the work function of GaN nanowire photocathodes decreases continuously with the increase of θCs, the “Cs-kill” phenomenon disappears, resulting in a lower work function (1.76 eV) than the conventional GaN planar photocathodes (1.82 eV). However, we find that the nanowire GaN photocathodes had a lower stability by calculating the adsorption energy. In addition, the surface atomic structures of both kinds of photocathodes are almost identical, which account for the similarity of their best adsorption sites. Our study is helpful to the growth of GaN nanowire materials in the future and can be used to guide the improvements of GaN-based equipment photoelectric efficiency.
Negative electron affinity (NEA) photocathodes have attracted a lot of interest over the last two decades due to their high quantum efficiency and low dark emission, which are desirable for night vision and other low-light applications. Recently, gradient-doping technique has shown promise to significantly improve the quantum yield of GaAs/AlGaAs heterojunction photocathodes by assisting electron diffusion toward the surface. In the present work, femtosecond pumpprobe transient reflectivity measurement has been used to study the ultrafast carrier dynamics in NEA GaAs/AlGaAs photocathodes. The research focuses on the comparison between a traditional, uniform-doped structure (1.7 μm p-GaAs (1×10<sup>19</sup> cm<sup>-3</sup>) / 0.7 μm p-Al<sub>0.57</sub>Ga<sub>0.43</sub>As (3×10<sup>18</sup> cm<sup>-3</sup>) / si-GaAs substrate) and a gradient-doped structure (0.1 μm pGaAs (1×10<sup>18</sup> cm<sup>-3</sup>) / 1.2 μm p-Al<sub>0.63</sub>Ga<sub>0.37</sub>As (doping level gradually changes from 1×10<sup>18</sup> cm<sup>-3</sup> to 1×10<sup>19</sup> cm<sup>-3</sup>) / 0.5 μm p-GaAlAs (1×10<sup>19</sup> cm<sup>-3</sup>) / si-GaAs substrate). Our result indicates that gradient doping not only leads to more efficient electron transportation but also results in better electron accumulation (i.e. higher concentration and longer lifetime) near device surface, a feature well-suited for photocathodes. Moreover, we have shown that pump-probe transient reflectivity measurement is able to offer a direct picture of electron diffusion inside NEA photocathodes, which can be of significant importance to device development.
In this paper, it is described the work principle and total structure and design of the laser-scanned measuring systerm for
the large diameter. Signal characteristics of scanning system is described,the circuit and method for boundaries detection
and drift errors. The feasibility of the system is tested by practice, the accuracy of boundary distinguishing is up to
±1μm,and continuous working errors up to ±5μm,measurement range up to 0~400mm. This system shows a better method for measuring large diameters.
To achieve high quantum efficiency and good stability has been a main direction to develop GaAs photocathode
recently. Through early research, we proved that variable doping structure is executable and practical, and has great
potential. In order to optimize variable doping GaAs photocathode preparation techniques and study the variable doping
theory deeply, a real-time quantum efficiency measurement system for GaAs Photocathode has been designed. The
system uses FPGA (Field-programmable gate array) device, and high speed A/D converter to design a high signal noise
ratio and high speed data acquisition card. ARM (Advanced RISC Machines) core processor s3c2410 and real-time
embedded system are used to obtain and show measurement results. The measurement precision of photocurrent could
reach 1nA, and measurement range of spectral response curve is within 400~1000nm. GaAs photocathode preparation
process can be real-time monitored by using this system. This system could easily be added other functions to show the
physic variation of photocathode during the preparation process more roundly in the future.
A new high spectrum difference technique was discussed in this paper, which use exploitation of quantum effects to
detect and warn low observable objects (include missile, stealth etc.). It is a new technique in photoelectric
reconnaissance area, and has many advantages such as wide radiation regions and greatly strengthens signal intensity. In
this paper radiation spectrum intensity, atmosphere attenuation and effect distance are studied. We found that high
intensity of stimulated emission spectrum can be obtained in atmosphere transmit window. So the sensitiveness and
distance of detect can be enormously improved.